1. Field of the Invention
The present invention relates to a rectilinear motion driving apparatus, which is applicable to facilities in a factory, various industrial machines or the like, and is driven by rotation of an electric motor, and converts a rotational motion into a rectilinear motion so that an output part can be linearly moved.
2. Description of the Related Art
Conventionally, in various technical fields, a ball screw has been used as a driving mechanism, which is driven by a rotation of electric motor so that an output part can be made rectilinear motion. This type of ball screw is constructed in a manner such that a female screw block is screwed onto an outer periphery of a screw shaft via many bearings, and the screw shaft is driven by a rotation of an electric motor so that the female screw block is linearly moved along an axial direction. However, in the ball screw, high accuracy is required for machining the screw shaft and the female screw block. For this reason, a problem arises such that a manufacture cost is high, and the price becomes high.
In order to solve the above problem, the inventor of the present invention has proposed a rectilinear motion driving apparatus disclosed in Japanese Unexamined Patent Publication (Kokai) No. 8-219248. The rectilinear motion driving apparatus is constructed in a manner such that a cylindrical coil spring is assembled between a cylinder having a slit formed in its axial direction and shaft at a predetermined angle to the axial direction, and an outer periphery of the cylinder is clamped by a ring to mount it.
However, this rectilinear motion driving apparatus has the following problem. More specifically, in the case where an inner surface of the cylinder has a low circularity (deviation from circular form) and is not formed into a true circle with high accuracy, a pressure contact force to the cylinder and the shaft varies in accordance with a rotational position of the cylindrical coil spring. As a result, it is difficult to obtain a stable rectilinear motion.
Moreover, a power transmission is made by a frictional force of the cylindrical coil spring from the cylinder to the shaft or from the shaft to the cylinder. However, in this case, their metallic curved surfaces contact each other. For this reason, a slip is generated between the cylindrical coil spring and the cylinder or shaft in driving. The slip is generated in an axial direction, and then, when the axial slip is repeated, a position of the cylindrical coil spring is biased in one direction by the above accumulative slip. As a result, a problem arises such that a motion stroke of the shaft gradually reduces.
Accordingly, an object of the present invention is to provide a rectilinear motion driving apparatus, which can be manufactured at a low cost as compared with a ball screw, and can carry out a stable rectilinear motion. Further, another object of the present invention is to provide a rectilinear motion driving apparatus, which can prevent a positioned bias of a cylindrical coil roller by an slip.
In order to achieve the above object, the present invention provides a rectilinear motion driving apparatus. In the rectilinear motion driving apparatus, a shaft is fitted into a cylindrical body in a coaxial state, and a plurality of cylindrical coil rollers is interposed between the cylindrical body and the shaft by press fit so as to roll at a predetermined inclined angle to the axial direction. Further, these cylindrical coil rollers contact with an outer peripheral surface of the shaft and an inner peripheral surface of the cylindrical body with a pressure contact force, and the cylindrical body or the shaft is rotated so that the shaft or the cylindrical body makes a rectilinear motion toward an axial direction. The cylindrical coil roller is arranged so as to roll on the outer peripheral portion of the shaft, and is arranged at an inclined angle to the axial direction of the shaft and the cylindrical body. Therefore, a pressure contact force to both surfaces of the cylindrical coil roller held between the inner peripheral surface of the cylindrical body and the outer peripheral surface of the shaft is set higher in the vicinity of the end portion of the cylindrical body or the shaft, or is set lower in the vicinity of the end portion of the cylindrical body or the shaft. By doing so, it is possible to correct a positional bias of the cylindrical coil roller by the accumulative slip.
In this case, in order to increase the pressure contact force to both surfaces of the cylindrical coil roller held between the inner peripheral surface of the cylindrical body and the outer peripheral surface of the shaft in the vicinity of the end portion of the cylindrical body or the shaft, the end portion of the cylindrical body may be formed thicker than the intermediate portion thereof. Further, in order to reduce the pressure contact force to both surfaces of the cylindrical coil roller held between the inner peripheral surface of the cylindrical body and the outer peripheral surface of the shaft in the vicinity of the end portion of the cylindrical body or the shaft, the end portion of the shaft may be provided with a taper portion formed in a manner such that the outer diameter of the end portion of the shaft is gradually reduced. Furthermore, the cylindrical body is formed of a pipe having a relatively thin wall thickness, and then, both ends of the pipe may be bound so that no deformation is generated in a radius direction.